40 PLANT-ANIMALS [CH. 



is an apparatus for that purpose : as well, of course, 

 as for other purposes. 



Arrived at the motor region, the nervous impulse 

 sets up an excitation in the protoplasm of that region. 

 As the result of this excitation, there arises a definite 

 modification in the hitherto uniform rate of elonga- 



o 



tion of the cells of the motor region. The cells of 

 one side grow faster than those of the other, and a 

 growth-curvature results by which the tip is carried 

 back to the vertical position. Such a mode of 

 nervous action is called a reflex. In every case, 

 in the simplest, unicellular organism and in the 

 highest animals, reflex action involves perception, 

 excitation, transmission to the motor region, excita- 

 tion of, and motor (or other) response by, that region. 

 All protoplasm, as we know it, contains the apparatus 

 required for this series of events, and evolution, as 

 we know it, has but resulted in the perfecting and com- 

 plicating of these reflex arcs. We may take the reflex 

 as the base or primal manifestation of all nervous 

 activity. In the reflexes of root and stem and leaf, 

 the stimuli light, gravity, etc. which induced them 

 give rise to the assumption by the root or stem 

 or leaf of a definite position with respect to the 

 direction whence the stimulus proceeds. Such 

 stimuli are therefore called directive stimuli. When 

 a fixed plant or animal responds to a directive 

 stimulus by a definite, purposeful curvature Ave de- 



